Series and parallel circuits: For components connected in series:

The idea

This statement introduces the rules that apply when components are connected in series. In a series circuit there is a single loop, so the current is the same at every point: if 0.30 A leaves the cell, 0.30 A passes through each component. The total potential difference of the supply is shared between the components, so the p.d.s across the components add up to the supply p.d.; for a 6.0 V supply with two identical lamps, each lamp takes 3.0 V. The total resistance of the circuit is the sum of the individual resistances, so adding a component increases the total resistance and reduces the current. These rules let you calculate an unknown current, p.d. or resistance in a series circuit, and they explain why removing one series component breaks the circuit and stops the current everywhere.

What earns the marks

• States that the current is the same at every point in a series circuit because there is only one path for charge to flow.
• States that the total potential difference of the supply is shared between the components in series, so the individual p.d.s sum to the supply p.d.
• States that the total resistance of components in series is the sum of their individual resistances.
• Applies the rules to calculate an unknown current, potential difference or resistance in a series circuit.
• Explains that adding a component in series increases total resistance and therefore reduces the current for a fixed supply p.d.

Common mistakes and how to put them right

• Believing current is shared between series components.

  Put it right by stating that current is the same through every component because there is only one path.

• Adding the potential differences of the supply and a component instead of recognising that component p.d.s sum to the supply p.d.

  Put it right by writing supply p.d. = sum of component p.d.s.

• Forgetting to add every resistance when finding total resistance.

  Put it right by listing all series resistances and summing them before using V = IR.

Exam tips

• Write the series rules as equations before substituting numbers: I is constant, V_total = V₁ + V₂ + ..., R_total = R₁ + R₂ + ....
• Check answers using V = IR for each component and confirm the p.d.s add to the supply value.
• If a question says a component is removed or added, state the effect on total resistance first, then on current.